1. Field of the Invention
The present invention generally relates to an automatic focusing (AF) system in a low-illumination setting and a method using the same, and more particularly, to an AF system and a method that is capable of achieving the goal of real-time focusing by selection of memory bits and tone adjustment.
2. Description of Related Art
AF technology is developed for the cameras or video cameras to maintain the lens within a focal length so as to generate a clear picture. As such, AF technology has been incorporated into digital cameras and video cameras.
There are several ways to implement a modern AF technology, and they are categorized into a passive mode and an active mode. The passive-mode AF measures the light reflected from an object before calculating where to move the lens. If the object is positioned in a low-illumination setting, an auxiliary focusing lamp is often used for light compensation. When the auxiliary focusing lamp is in use, a color beam is projected onto and reflected back by the object. Consequently, a sufficient amount of the light could be reflected back from the object through which a focal length could be analyzed before the position of the lens could be adjusted.
The active-mode AF technology allows the camera or the video camera to issue a supersonic wave or an infrared ray before measuring the distance between the object and the camera according to the issued supersonic wave or infrared ray reflected from the object. Particularly, the propagated time or the reflective angle is used for obtaining the distance so that the camera may adjust the focal length.
With the advance of the technology, some cameras or video cameras are equipped with AF technology through real-time capturing an image by an image sensor and previewing the image through a LCD screen in the camera or video camera, which is an example of live view automatic focusing. Such AF technology is configured to accomplish a composition and the automatic focusing simultaneously with the camera or video camera installed with a processor capable of processing a large volume of images. Through the processor dedicated to processing the images, the camera or video camera may adjust its focus by comparing the image captured by the image sensor in a real-time fashion via an image definition computation.
Please refer to FIG. 1 of a flow chart illustrating a conventional AF procedure. The AF procedure includes driving a stepping motor to move a focusing lens (step S101). When the focusing lens is moved to a specific position, an image is captured by a lens array (step S103). In step S105, one object's definitions at several preset positions are calculated and the corresponding focal lengths can be obtained as well. The procedure then determines whether it has sufficient data or not (step S107) to determine a most suitable focal position. If the data is not yet sufficient, the conventional procedure repeats the steps of moving the focusing lens through the stepping motor, capturing the image, and calculating the definitions. If the data is sufficient, the procedure is configured to estimate which position of the focusing lens is associated with a finest definition. The position associated with finest definition serves as the basis for the determination of the most suitable focal position that tends to be associated with a clearest image (step S109). After that, the lens array is moved to that position to accomplish the entire procedure (step S111).
However, the image captured by the previously discussed AF procedure may be associated with insufficient illumination, rendering the automatic focusing to be inaccurate.
FIG. 2 shows a block diagram of a conventional digital camera. The conventional digital camera includes circuits for automatic white balance, automatic exposure, and automatic focusing. The camera captures images via a lens 20, and the images are received by an image sensor 21 and converted into digital signals. Those digital signals are processed by an image pre-processing unit 22 before their color temperatures are adjusted by an automatic white-balance unit 23. An image signal processor 24 then previews, photographs, and/or stores the images. During the course of capturing the images, an automatic exposure unit 27 is necessary for the calculation of an exposure value while an automatic focusing unit 28 is configured to obtain an accurate imaging distance. Thereafter, the information including the exposure value and the imaging distance is used for controlling a lens motor via a motor driver 29 before any properly focused image could be obtained.
However, the capturing of the image by the image sensor, the image processing, the image preview, and the image storage may be associated with retrievals of the higher memory bits due to the hardware design at the expense of focusing accuracy. In a low-illumination setting, the retrieval of the higher memory bits may cause a loss of details of the image captured, increasing the noise and further undermining the focusing accuracy.